Data encryption systems have been devised which are used to digitally reproduce an analog signal without substantial distortion or loss. Typically, such systems are based upon the Nyquist theorem which states that an analog bandwidth-limited signal can be reproduced exactly by sampling the signal at a rate at least twice the upper bandwidth limit of the signal to create a staircase approximation of the analog signal and by passing the staircase approximation through an ideal low-pass filter having a cutoff frequency equal to the upper bandwidth limit. In accordance with this theorem, prior systems have sampled an analog signal at a constant rate which is at least twice the highest anticipated frequency of the signal to create the staircase approximation. In the field of audio signal reproduction where the upper bandwidth limit is generally assumed to be 20 kilohertz, a sampling frequency of 44.1 kilohertz is utilized to encode the analog signal.
Such systems typically store the staircase approximation as a series of digital words on or in a storage medium, such an audio compact disc, a hard disk or an optical or video disc.
The analog signal is then reproduced by retrieving the digital words at a constant rate, converting the series of digital words into the staircase approximation of the analog signal by means of a D/A converter and by passing the staircase approximation through an ideal low-pass filter having a cut-off frequency equal to 20 kilohertz.
The foregoing technique for digitally reproducing an analog signal has been found to be disadvantageous in that a large amount of digital words are required to encode an analog signal of given duration.
Other types of systems for digitally reproducing an analog signal include Cherry et al U.S. Pat. No. 3,324,237, Kitamura et al U.S. Pat. No. 4,568,912, Cherry et al U.S. Pat. No. 3,299,204, Stapleton U.S. Pat. No. 3,449,742, Jordan U.S. Pat. No. 4,308,585, Kitamura U.S. Pat. No. 4,370,643 and Tsuchiya et al U.S. Pat. No. 4,348,699.
The Cherry et al '237 patent, in particular, discloses a television transmission system in which an analog video signal is sampled at a six megahertz rate to create a series of equally-spaced digital samples which are analyzed to derive a bandwidth associated with each sample. A predetermined number of samples defining a first portion of the video signal are then analyzed to determine whether the bandwidth associated with each sample is less than a particular frequency. If so, all but one of the predetermined number of samples are dropped and the last sample is retained. On the other hand, if less than all of the bandwidths associated with the samples are less than the particular frequency, then only some or none of the samples are dropped, with the remainder being retained. Further predetermined numbers of samples defining subsequent portions of the video signal are processed in a like way to develop a signal containing a number of unequally spaced samples which define the original video signal. This system, however, can only sample at frequencies which are an integer submultiple of the original six megahertz frequency, and hence resolution is limited and oversampling is bound to occur under usual circumstances.
A still further type of digital method and system for reproducing an analog signal is disclosed and claimed in the cross-referenced Podolak et al patent application identified above. This system detects the upper bandwidth limit of the analog signal to be reproduced, samples the analog input signal at a variable sampling rate determined in accordance with the detected upper bandwidth limit to define a sampled approximation of the input analog signal comprising a series of voltage levels and converts the sampled approximation into a series of digital signals or words each of which contains bits defining a voltage level and the sampling rate at which such voltage level was derived. These signals or words are then stored in a storage medium.
The analog signal is reproduced by decoding apparatus which sequentially retrieves the digital words from the storage medium, converts the digital words into the sampled approximation of the analog input signal by means of a D/A converter and filters the sampled approximation using a low-pass filter having a cut-off frequency determined in accordance with the encoded sampling rate of each word.
This system also encodes periods of silence during which the analog signal is at a substantially zero level as one or more digital words which together indicate the duration of such period, rather than encoding a substantially greater number of digital signals each of which represent a substantially zero voltage level for a portion of the silence period.
The foregoing system has been found to result in a substantial reduction in the amount of digital words required to encode an analog signal as compared with the prior art.
Moreover, it has been found that the foregoing system is also effective for use in those situations where no storage medium is utilized, such as when it is desired to transmit an audio signal in digital form over a communication medium.
A further example of a silence detection and encoding system is disclosed in IBM Technical Disclosure Bulletin, Vol. 20, No. 4, September 1977. When silence in an input signal is sensed, a counter is enabled to create a digital indication of the silence duration. The digital indication is stored with other digital signals representing nonsilence portions of the input signal.